1. Technical Field
The disclosure relates to a heat spreader/lid, especially for microelectronic flip-chip packaging, in which the heat spreader has an underside surface that includes at least one curvilinear contour. The curvilinear contour improves the heat dissipation and mechanical stress of a thermal interface between the heat spreader or lid and the circuit chip. The disclosure also relates to a microelectronic package that includes the heat spreader/lid.
2. Discussion of the Background
Heat dissipation from a circuit chip to a heat spreader/lid is important for microelectronic packages and depends on the thermal resistance of package components. The thermal resistance is determined by the thermal conductivity, the thickness of materials and the degree of wetting at joining interfaces. The heat dissipation increases with increasing thermal conductivity and the degree of wetting, and decreasing thickness.
The major function of thermal interface materials (TIM) is to dissipate heat from the chip by thermally and mechanically linking the chip to the heat spreader. The thermal interface between the chip and the heat spreader usually has significantly smaller thermal conductivity than any other component of a microelectronic package. The thermal performance of the TIM can be improved by introducing materials with higher thermal conductivity and smaller TIM thickness. Inorganic fillers (metals and mineral oxides) are mixed with TIM gels and pastes to increase the thermal conductivity or decrease the thermal resistance of TIM. As TIM bondline decreases, the mechanical stress increases and can become a design constraint.
A soft or low-modulus TIM adhesive is preferred for flip chip plastic ball grid array (FCPBGA) packages to reduce overall mechanical stresses that are governed by the coefficient of thermal expansion (CTE) mismatched chip, underfill and organic substrate structure as shown in FIG. 1. At the same time, the TIM should be resistant to large or permanent deformations during thermal cycling that could result in crack initiation and propagation. TIM greases, which are very soft and ultra low modulus are not viable in FCPGBA packages since these materials are mobile during thermal cycling, resulting in voids and severe material loss due to the outward pumping of the paste. Consequently, effective heat dissipation is reduced and an operating chip can overheat and even fail. There is a balance that TIM thickness must achieve to satisfy both the heat dissipation and mechanical stress management requirements for chip packages.
U.S. Pat. No. 6,091,603, incorporated herein by reference in its entirety, generally discloses that thermal resistance may be reduced by introducing customized lid understructure to reduce the amount of thermally conductive material. In particular, the smaller the TIM volume, the smaller the thermal resistance. U.S. Pat. No. 6,472,762 generally discloses the use of a heat spreader with high coefficient of thermal expansion. U.S. Pat. No. 6,784,535 generally discloses a composite lid consisting of at least two materials. However, the above-mentioned disclosures do not indicate the use of curvilinear contours on a heat spreader underside to locally control the bondline for improvements to both heat dissipation and mechanical stress management in the TIM.